Titanium dioxide (TiO2) is an important pigment in the manufacture of paints, plastics, and coatings. There has been a considerable research effort to make titanium dioxide pigments with desirable properties (i.e., fine particle size, gloss and durability).
One method of manufacturing titanium dioxide is by reacting titanium tetrachloride (TiCl4) with oxygen. This reaction is initiated by heating the gaseous reactants (TiCl4 and oxygen) to temperatures of preferably between 650 and 1200xc2x0 C. Some prior art references describe reducing these heating requirements by using multi-stage introduction of TiCl4 or oxygen into the reaction zone.
The prior art describes modifying the TiCl4 and oxygen reaction with chemicals to produce pigments with desirable properties. For example, the prior art describes adding aluminum trichloride (AlCl3) with TiCl4 to promote rutile titanium dioxide formation. AlCl3 addition alters the surface chemistry of titanium dioxide; enriching the surface of the titanium dioxide with aluminum (present as the oxide and/or titanate).
It is known that the reaction between TiCl4 and oxygen is highly exothermic and temperatures from the reaction mass range between about 1200 and about 2000xc2x0 C. These high temperatures can lead to undesired growth and agglomeration of titanium dioxide particles reducing pigmentary value. This undesired growth of titanium dioxide is exacerbated at high production rates, high temperatures and high pressures.
In conventional manufacturing processes, the undesired growth of titanium dioxide is prevented by rapidly cooling the reaction mass to below 600xc2x0 C. This is accomplished by passing the reaction products through a conduit, xe2x80x9cfluexe2x80x9d, which is externally cooled by water. The hot pigment tends to stick to the flue walls causing a build up. This build up can be reduced or eliminated by introducing scouring particles or scrubs materials. Some examples of scrubs material include NaCl, KCl, sand, and the like. The cooled titanium dioxide is separated from the gases by filtration and then dispersed in water for further processing.
TiO2 dioxide pigment properties, such as iron oxide undertone (IOU) and gloss, are a function of particle size distribution and particle agglomeration, respectively. When highly agglomerated TiO2 is formed, it must be milled in an expensive, energy-intensive process such as sand-milling or micronizing to achieve the desired particle size. The energy consumption and intensity of grinding or milling agglomerates depends not only on the number of agglomerates present but also on their strength, that is, how strongly the primary or individual titanium dioxide particles are bonded to each other.
One way to reduce particle size and agglomerates is to add a silicon halide (i.e. silicon tetrachloride). The reaction between silicon tetrachloride (SiCl4) and oxygen results in the formation of silica. Silica reduces the sintering rate of titania and results in smaller particles and fewer agglomerates with weak bonds.
Unfortunately, silicon halide addition promotes unwanted anatase formation in titanium dioxide. Out of the two commercially significant crystal modifications of titanium dioxide (i.e., anatase and rutile), the anatase form is photochemically more active and hence, less durable. Even 1% anatase in rutile titanium dioxide is detrimental to the durability of the pigment. Rutile is further preferred due to its higher refractive index. Thus, it is desirable to produce essentially anatase-free titanium dioxide with a rutile content of at least 99.8% or higher.
The anatase promoting effect of silicon compounds has been countered in the prior art by using high levels of aluminum chloride. For example, TiCl4 is premixed with silicon species and volatile alumina (i.e., AlCl3) before entering the reaction zone. It takes temperatures of between 1000 and 1200xc2x0 C., to form about 90% rutile titanium dioxide in this process. However, aluminum halide consumption is increased causing a higher production cost.
Premixing SiCl4 and AlCl3 with TiCl4 to make high surface area titania in hydrogen flames is described in the prior art. The pigment produced is used for catalytic, sun-screen, and cosmetic applications.
Adding certain conditioning agents like silicon and aluminum halides is also described in the prior art. The use of these conditioning agents can be minimized by using multiple stages of TiCl4 addition. However, the conditioning agents are premixed only in the first stage. This scheme, with SiCl4, results in smaller titania particle size and greater than 97% rutile content.
Some prior art references describe improving particle size and titania tint tone by separately adding between 0.01 and 8% SiCl4 to the TiCl4 stream and between 0.00001 and 4% alkali salt to the oxygen stream. Some silicon sources used are silicon halides, silanes, alkylalkoxysilanes, alkylsilic esters or ethers, and derivatives of silicic acid.
Delayed introduction of silicon compounds into the reactor is also described in the prior art. In this case, the silicon compound is introduced separately after the TiCl4 and oxygen have been introduced into the reactor. This process results in smaller titanium dioxide particles.
The prior art describes silicon compound addition after the reaction of TiCl4 and oxygen is substantially complete. In this case, after TiCl4 and oxygen are reacted, the resulting TiO2 is coated by suspending pigment in a gas stream and then reacting steam and silicon or aluminum compounds on the surface of titanium dioxide.
Other prior art references describe adding silicon halides into the flue after the reaction between TiCl4 and oxygen is initiated. This overcomes the anatase promoting effects of silicon and yields smaller particles. Since the silicon compound is added after the reaction is substantially complete, higher amounts of silicon compound are needed to achieve the same degree of particle size reduction than in the case where silicon compounds are added prior to the reaction. Moreover, the delayed addition of silicon halide leads to most of the silica residing on the outer surface of titania particle or as discrete silica particles. This alters the particle surface chemistry and can increase difficulty in dispersing the pigment in an aqueous medium.
Based on the foregoing, the need exists for processes of producing substantially anatase-free titanium dioxide (rutile content of at least 99.8%) having optimum pigmentary size at high production rates and operating pressures. Premixing silicon compounds and titanium tetrachloride together and then reacting them with oxygen at pressures in excess of 55 psig provides a means of manufacturing substantially-anatase free titanium dioxide with reduced particle size and controlled surface chemistry. This is achieved without increasing processing temperature or aluminum chloride feed and reduces or eliminates the need for scrubs material.
The present invention provides methods of producing substantially anatase-free titanium dioxide by introducing a silicon compound into the TiCl4 stream to form an admixture, before reaction with oxygen. The admixture which comprises titanium tetrachloride and the silicon compound is introduced into a reaction zone with oxygen to produce the substantially anatase-free titanium dioxide, where the reaction zone has a pressure of greater than 55 psig.
In one embodiment, the present invention provides a method of producing substantially anatase-free titanium dioxide, comprising: a) mixing titanium tetrachloride with silicon tetrachloride and aluminum trichloride to form an admixture; and b) introducing the admixture and oxygen into a reaction zone to produce the substantially anatase-free titanium dioxide, wherein the reaction zone has a pressure of about 70 psig.
In another embodiment, the present invention provides a method of producing substantially anatase-free titanium dioxide in a multi-stage reactor having a plurality of reaction zones, the method comprising a) mixing titanium tetrachloride with a silicon compound to form one or more admixtures; and introducing one admixture or a portion thereof, and oxygen into each reaction zone of the multistage reactor to produce the substantially anatase-free titanium dioxide, wherein each reaction zone is at a pressure greater than 55 psig.
The methods of the present invention are different from prior art where mixing of TiCl4 with a silicon compound resulted in some anatase titanium dioxide formation. The present invention includes reacting a silicon compound at higher pressures (greater than 55 psig). Using high pressure with a silicon compound results in substantially anatase-free TiO2 (rutile content of at least 99.8%) with reduced particle size, less silicon and aluminum chloride use throughout the process, reduces or eliminates the need for scrubs, without increasing operating temperature.
The reduction in particle size results in improved IOU and gloss. Further, the energy consumption and intensity of milling to achieve product size and gloss targets are reduced than in the process without use of the present invention. It has also been found that this improvement permits higher titanium dioxide output than would otherwise be possible while maintaining the desired particle size targets. It has further been found that the need for scrubs introduced into the cooling conduit is reduced or eliminated.
For a better understanding of the present invention together with other and further embodiments, reference is made to the following description taken in conjunction with the examples, the scope of which is set forth in the appended claims.